Water-dilutable coating agent for preparing the base layer of a multilayer coating

ABSTRACT

Basecoating compositions for preparing multilayered protective and/or decorative coatings comprise aqueous dispersions which contain 
     (a) as film-forming material one or more polyurethane resins having an acid number of from 5 to 70, which has been prepared by preparing from 
     (A) liner polyetherdiols and/or polyesterdiols having a molecular weight of from 400 to 3,000 
     (B) diisocyanates and 
     (C) compounds which contain two groups which are reactive toward isocyanate groups, one or more of the compounds used as component C having one or more groups capable of anion formation which have preferably been neutralized before the reaction with a tertiary amine, 
      an intermediate which as terminal isocyanate groups and whose free isocyanate groups have subsequently been reacted with a polyol which contains three or more hydroxyl groups, preferably a triol, 
     (b) pigments and 
     (c) further customary additives.

The invention relates to a basecoating composition for preparingmultilayered, protective and/or decorative coatings on substratesurfaces,

comprising an aqueous dispersion which contains

(a) as film-forming material one or more polyurethane resins having anacid number of from 5 to 70, which has been prepared by preparing from

(A) Linear polyetherdiols and/or polyesterdiols having a molecularweight of from 400 to 3,000

(B) diisocyanates and

(C) compounds which contain two groups which are reactive towardisocyanate groups, one or more of the compounds used as component Chaving one or more groups capable of anion formation which havepreferably been neutralized before the reaction with a tertiary amine,

an intermediate which has terminal isocyanate groups and whose freeisocyanate groups have subsequently been reacted with

(D) further compounds which contain groups reactive toward isocyanategroups,

(b) pigments and

(c) further customary additives.

It is known in particular in automotive coating but also in other areaswhere coatings of pleasing decorative effect and at the same time highcorrosion inhibition are desirable to provide substrates with aplurality of superposed coating layers.

Multilayer coatings are preferably applied by the basecoat-clearcoatprocess, wherein a pigmented basecoat is applied first and, after ashort flashoff time, is overpainted wet-on-wet (i.e. without priorbaking) with clearcoat. Basecoat and clearcoat are subsequently bakedtogether.

The basecoat-clearcoat process has attained particularly greatimportance in the application of automotive metallic effect coatings.

Economic and ecological reasons were behind attempts to use waterbornebasecoating compositions in the preparation of multilayer coatings.

Coating agents for preparing basecoats for multilayer automotivecoatings must be processable by the currently customary efficientwet-on-wet process, i.e. they must be overcoatable after a very shortpredrying time with a (transparent) topcoat without showing unwantedsigns of redissolving.

In the development of coating agents for basecoats of metallic effectcoatings, it is additionally necessary to solve other problems. Themetallic effect depends crucially on the orientation of the metalpigment particles in the paint film. A metallic effect basecoat which isprocessable wet-on-wet must accordingly produce paint films in which themetal pigments are present after application in a favorable spatialorientation and in which this orientation is rapidly fixed in such a waythat it cannot be disturbed in the course of the rest of the coatingprocess.

In the development of water-dilutable systems which are to meet therequirements described above, difficult problems arise due to thespecial physical properties of water, and to date there exist only fewwater-dilutable coating systems which can be used as baecoatingcompositions in the abovementioned sense.

For instance, U.S. Pat. No. 4,558,090 discloses coating agents forpreparing the base layer of multilayer coatings, which comprise anaqueous dispersion of a polyurethane resin having an acid number of5-70. The aqueous polyurethane dispersion which, in addition to thebinder, may also contain pigments and customary additives and if desiredfurther binder components is prepared by reacting

(A) a linear polyetherdiol and/or polyesterdiol having terminal hydroxylgroups and a molecular weight of from 400 to 3,000 with

(B) a diisocyanate and

(C) a compound which has two groups which are reactive toward isocyanategroups and one or more groups capable of anion formation, the groupcapable of anion formation having been neutralized before the reactionwith a tertiary amine,

to give an intermediate having the terminal isocyanate groups,converting the intermediate contained (sic) from (A), (B) and (C) into apredominantly aqueous phase and

(D) reacting the isocyanate groups still present with a di- and/orpolyamine having primary and/or secondary amino groups.

The coating agents disclosed in U.S. Pat. No. 4,558,090 are highlysuitable for preparing the base layer of multilayer coatings, but theyare unsuitable for practical use, in particular in mass productioncoating processes, since the rapidly drying coating agents are so highlyadherent to the application equipment used (for example paint sprayguns; automatic, electrostatically aided high-rotation units and thelike) that they can only be removed with great difficulty. As aconsequence, it is impossible to change the applied coating systems, forexample to change the color, with the high speed which is veryfrequently required in particular in automotive mass production coating.

It is an object of the present invention to develop aqueous dispersionswhich can be used as basecoating compositions for preparing multilayeredprotective and/or decorative coatings on substrate surfaces and whichmeet all the abovementioned requirements of a basecoating compositionand can also be processed without problems in the application equipmentused.

We have found that, surprisingly, this object is achieved by usingaqueous dispersions as defined in the first part of claim 1, wherein thepolyurethane resin has been prepared by reacting the intermediateobtained from (A), (B) and (C) with a polyol, preferably a triol, whichcontains three or more hydroxyl groups, and converting the reactionproduct thus obtained into the aqueous phase.

It is surprising and was not foreseeable that the difficulties in theremoval of paint residues in the application equipment due to inherentlydesirable properties (rapid drying of the applied wet film, difficultredissolving of the dried film) can be solved by using the aqueouspolyurethane dispersion according to the invention and that no decrementin the quality of the completed multilayer coating need be incurred.

The dispersions according to the invention are obtained by reactingcomponents (A), (B) and (C) to give an intermediate which has terminalisocyanate groups. The reaction of components (A), (B) and (C) takesplace in the conventional manner of organic chemistry, preferably instages (for example formation of a first intermediate from components(A) and (B), which is then reacted with (C) to give a secondintermediate). But it is also possible to react components (A), (B) and(C) simultaneously.

The reaction is preferably carried out in solvents which are inerttoward isocyanate groups and water-miscible. It is advantageous to usesolvents which, in addition to having the properties described above,are also good dissolvers for the polyurethanes prepared and can beeasily separated from aqueous mixtures. Particularly highly suitablesolvents are acetone and methyl ethyl ketone.

Component (A) can in principle be any diol customary in the preparationof polyurethane-based coating agents. Suitable polyether diols conformto the general formula: ##STR1## where R is hydrogen or lower alkyl withor without various substituents, n is 2-6 and m is 10-50 or higher.Examples are poly(oxytetramethylene) glycols, poly(oxyethylene) glycolsand poly(oxypropylene) glycols.

The preferred polyalkylene ether polyols are poly(oxypropylene) glycolshaving a molecular weight within the range from 400 to 3,000.

Polyesterdiols can likewise be used as polymeric diol component(component A) in the invention. The polyesterdiols can be prepared byesterifying organic dicarboxylic acids or the anhydrides thereof withorganic diols. The dicarboxylic acids and the diols can be aliphatic oraromatic dicarboxylic acids and diols.

The diols used for preparing the polyesters include alkylene glycolssuch as ethylene glycol, butylene glycol, neopentylglycol and otherglycols such as dimethylolcyclohexane.

The acid component of the polyester chiefly comprises low molecularweight dicarboxylic acids or anhydrides thereof of 2 to 18 carbon atomsin the molecule.

Suitable acids are for example phthalic acid, isophthalic acid,terephthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid,adipic acid, azelaic acid, sebacic acid, maleic acid, glutaric acid,hexachloroheptanedicarboxylic acid and tetrachlorophthalic acid. Inplace of these acids it is also possible to use their anhydrides,provided they exist.

Furthermore, in the invention it is also possible to use polyesterdiolswhich are derived from lactones as component (A). These products areobtained for example by reacting an ε-caprolactone with a diol. Productsof this type are described in U.S. Pat. No. 3,169,945.

The polylactonepolyols which are obtained by this reaction have aterminal hydroxyl group and recurring polyester portions which arederived from the lactone. These recurring molecular portions can conformto the formula ##STR2## where n is preferably 4 to 6 and the substituentis hydrogen, alkyl, cycloalkyl or alkoxy, no substituent containing morethan 12 carbon atoms and the total number of carbon atoms in thesubstituent on the lactone ring not exceeding 12.

The lactone used as starting material can be any desired lactone or anydesired combination of lactones, although this lactone should containnot less than 6 carbon atoms in the ring, for example from 6 to 8 carbonatoms, and not less than 2 hydrogen substituents should be present onthe carbon atom which is bonded to the oxygen group of the ring. Thelactone used as starting material can be represented by the followinggeneral formula: ##STR3## where n and R have the previously mentionedmeanings.

The lactones preferred in the invention for the preparation of thepolyesterdiols are the caprolactones, where n is 4. The most preferablelactone is the unsubstituted ε-caprolactone, where n is 4 and all Rsubstituents are hydrogen. This lactone is particularly preferred sinceit is available in large quantities and produces coatings havingexcellent properties. It is also possible to use various other lactonesindividually or in combination.

Examples of aliphatic diols suitable for the reaction with the lactoneinclude ethylene glycol, 1,3-propanediol, 1,4-butanediol anddimethylolcyclohexane.

Component (B) for the preparation of the polyurethane dispersion can beany desired organic diisocyanate. Examples of suitable diisocyanates aretrimethylene diisocyanate, tetramethylene diisocyanate, pentamethylenediisocyanate, hexamethylene diisocyanate, propylene diissocyanate,ethylethylene diisocyanate, 2,3-dimethylethylene diisocyanate,1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate,1,4-cyclohexylene diisocyanate, 1,2-cyclohexylene diisocyanate,1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-toluylenediisocyanate, 2,6-toluylene diisocyanate, 4,4'-biphenylene diisocyanate,1,5-naphthylene diisocyanate, 1,4-naphthylene diisocyanate,1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclohexane,bis(4-isocyanatocyclohexyl)methane, bis(4-isocyanatophenyl)methane,4,4'-diisocyanatodiphenyl ether and2,3-bis(8-isocyanatooctyl)-4-octyl-5-hexyl-cyclohexene.

Component (C) comprises compounds which contain two groups reactivetowards isocyanate groups, one or more of the compounds used ascomponent (C) having one or more groups capable of anion formation whichhave preferably been neutralized before the reaction with a tertiaryamine.

By setting a certain mixing ratio between the compounds which containgroups capable of anion formation and the compounds which are free ofthese groups it is possible to control the proportion of ionic groups inthe polyurethane molecule.

Suitable groups which react with isocyanate groups are in particularhydroxyl groups. The use of compounds which contain primary or secondaryamino groups can have an adverse effect on the above-describedprocessability of the dispersions. The nature and amount of anyamino-containing compounds to be used can be determined by the skilledworker by means of simple routine studies.

Suitable groups capable of anion formation are in particular carboxyland sulfone groups. These groups can be neutralized before the reactionwith a tertiary amine to avoid any reaction with the isocyanate groups.

Examples of compounds which contain two or more groups which react withisocyanate groups and one or more groups capable of anion formation aredihydroxypropionic acid, dimethylolpropionic acid, dihydroxysuccinicacid and dihydroxybenzoic acid. Also suitable are polyhydroxy acidsaccessible by oxidation of monosaccharides, for example gluconic acid,saccharic acid, mucic acid, glucuronic acid and the like.Amino-containing compounds are for example α, -diaminovaleric acid,3,4-diaminobenzoic acid, 2,4-diaminotoluene-5-sulfonic acid,4,4'-diaminodiphenyl ether sulfonic acid and the like.

Suitable tertiary amines for neutralizing anionic groups are for exampletrimethylamine, triethylamine, dimethylaniline, diethylaniline,triphenylamine and the like.

Compounds which have two groups which are reactive toward isocyanategroups but are free of groups capable of anion formation can be forexample low molecular weight diols or diamines having primary orsecondary amino groups.

The isocyanato-containing intermediate formed from (A), (B) and (C) isreacted with the polyol which contains three or more hydroxyl groups,the consequence of which is most probably a chain lengthening andpossibly even a branching of the binder molecule.

In this reaction care must be taken to ensure that no crosslinkedproducts are obtained. This can be achieved for example by adding anamount of polyol adapted to the isocyanate group content of theintermediate from (A), (B) and (C) and to the reaction conditions.

In principle, any polyol which contains three or more hydroxyl groupsand can be reacted with the intermediate obtained from (A), (B) and (C)in such a way as to form no crosslinked products is suitable forpreparing the polyurethane dispersion according to the invention.Examples are trimethylolpropane, glycerol, erythritol, mesoerythritol,arabitol, adonitol, xylitol, mannitol, sorbitol, dulcitol, hexanetriol,(poly)pentaerythritol and the like.

Very particularly good results can be obtained when usingtrimethylolpropane as the polyol.

It is also conceivable that by reacting all the four components, namely(A), (B), (C) and polyol, simultaneously it is possible to prepareuncrosslinked polyurethanes which can be processed to give usablebasecoating compositions.

After reaction of the intermediate obtained from (A), (B) and (C) withthe polyol component, which has preferably been carried out in awater-miscible solvent which is inert toward isocyanate groups, which isa good solvent for the polyurethane formed and which is readilyseparable from aqueous mixtures, for example acetone or methyl ethylketone, and, if still to be carried out, neutralizing the groups capableof anion formation, the reaction product is converted into an aqueousphase. This can be done for example by dispersing the reaction mixturein water and distilling off the organic solvent portions which boilbelow 100° C.

For the purposes of the present invention, aqueous phase is water whichcan additionally contain organic solvents. Examples of solvents whichcan be present in the water are heterocyclic, aliphatic or aromatichydrocarbons, monohydric or polyhydric alcohols, ethers, esters andketones, for example N-methylpyrrolidone, toluene, xylene, butanol,ethylglycol and butylglycol and acetates thereof, butyldiglycol,ethylene glycol dibutyl ether, ethylene glycol diethyl ether, diethyleneglycol dimethyl ether, cyclohexanone, methyl ethyl ketone, acetone,isophorone or mixtures thereof.

After the pH of the resulting polyurethane dispersion has been checkedand if necessary been adjusted to a value within the range from 6 to 9,the dispersion forms the basis of the coating agents according to theinvention into which the other constituents, for example additionalbinders, pigments, organic solvents and assistants, are homogeneouslyincorporated by dispersing, for example by means of a stirrer ordissolver. Thereafter the pH is checked again and if necessary adjustedto a value within the range from 6 to 9, preferably from 7.0 to 8.5.Also the solids content and the viscosity are set to values adapted tothe particular application conditions.

The ready-to-use coating agents generally have a solids content of from10 to 30% by weight, and their efflux time in the ISO cup 4 is from 15to 30 seconds, preferably from 18 to 25 seconds. Their water content isfrom 60 to 90% by weight, and their organic solvent content from 0 to20% by weight, in each case based on the total coating agent.

The advantageous actions of the coating compositions according to theinvention can be ascribed to the use of the aqueous polyurethanedispersion described above.

In many cases it is desirable to improve the properties of the resultingsurface coatings in a controlled manner by including further bindersystems in the basecoating composition.

The basecoating compositions according to the invention advantageouslycontain as additional binder component a water-dilutable melamine resinin an amount of from 1 to 80% by weight, preferably from 20 to 60% byweight, based on the solids content of the polyurethane dispersion.

Water-soluble melamine resins are known per se and are widely used. Theyare etherified melamine-formaldehyde condensation products. Their watersolubility depends not only on the degree of condensation, which shouldbe as low as possible, but also on the etherification component, onlythe lowest members of the alkanol or ethylene glycol monoether seriesgiving water-soluble condensates. The hexamethoxymethylmelamine resinsare of the greatest importance. If solubilizers are used, evenbutanol-etherified melamine resins can be dispersed in the aqueousphase.

It is also possible to incorporate carboxyl groups in the condensate.Transetherification products of highly etherified formaldehydecondensates with hydroxycarboxylic acids are water-soluble afterneutralization on account of their carboxyl group and can be used ascrosslinker component in the coating agents according to the invention.

In place of the melamine resins described it is also possible to useother water-soluble or water-dispersible amino resins, for examplebenzoguanamine resins.

When the basecoating composition according to the invention contains amelamine resin, it can advantageously additionally contain, as a furtherbinder component, a water-dilutable polyester resin and/or awater-dilutable polyacrylate resin, the weight ratio of melamine resin:polyester/polyacrylate resin being from 2:1 to 1:4 and the total amountof melamine resin and polyester/polyacrylate resin, based on the solidscontent of the polyurethane dispersion, ranging from 1 to 80% by weight,preferably from 20 to 60% by weight.

Water-dilutable polyesters are those having free carboxyl groups, i.e.polyesters of high acid number.

There are in principle two known methods for incorporating the necessarycarboxyl groups in the resin system. The first method comprisesterminating the esterification at the desired acid number. Afterneutralization with bases, the polyesters thus obtained are soluble inwater and undergo film formation in the course of baking. The secondmethod comprises forming partial esters of dicarboxylic orpolycarboxylic acids with high-hydroxyl polyesters of low acid number.This reaction is customarily carried out with the dicarboxylic acids inanhydride form, which reacts under mild conditions with the hydroxylcomponent by forming a free carboxyl group.

The water-dilutable polyacrylate resins, like the polyester resinsdescribed above, contain free carboxyl groups. They generally compriseacrylic or methacrylic copolymers, and the carboxyl groups come from theacrylic or methacrylic acid portions.

Suitable crosslinking agents include blocked polyisocyanates. Accordingto the invention, it is possible to use any desired polyisocyanate wherethe isocyanate groups have been reacted with a compound so that theblocked polyisocyanate formed is resistant to hydroxyl groups at roomtemperatures but reacts at elevated temperatures, in general within therange from about 90° to 300° C. In the preparation of the blockedpolyisocyanates it is possible to use any desired organicpolyisocyanates which are suitable for the crosslinking. Preference isgiven to those isocyanates which contain from about 3 to about 36, inparticular from about 8 to 15, carbon atoms. Examples of suitablediisocyanates are the abovementioned diisocyanates (component B).

It is also possible to use polyisocyanates of higher isocyantesfunctionality. Examples thereof are tris(4-isocyanatophenyl)methane,1,3,5-triioscyanatobenzene, 2,4,6-triisocyanatotoluene,1,3,5-tris(6-isocyanatohexyl)biuret,bis(2,5-diisocyanato-4-methylphenyl)methane and polymericpolyisocyanates, such as dimers and trimers of diisocyanatotoluene. Itis also possible to use mixture of polyisocyanates.

The organic polyisocyanates which are suitable for use as crosslinkingagents in the invention can be prepolymers which are derived for examplefrom a polyol, including a polyetherpolyol or a polyesterpolyol. To thisend it is known to react polyols with excess polyisocyanates to formprepolymers having terminal isocyanate groups. Examples of polyols whichcan be used for this purpose are simple polyols, such as glycols, forexample ethylene glycol and propylene glycol, and other polyols, such asglycerol, trimethylolpropane, hexanetriol and pentaerythritol,furthermore monoethers, such as diethylene glycol and dipropyleneglycol, as well as polyethers which are adducts of such polyols andalkylene oxides. Examples of alkylene oxides which are suitable forpolyaddition onto these polyols to form polyethers are ethylene oxide,propylene oxide, butylene oxide and styrene oxide. These polyadditionproducts are generally referred to as polyethers having terminalhydroxyl groups. They can be linear or branched. Examples of suchpolyethers are polyoxyethylene glycol having a molecular weight of1,540, polyoxypropylene glycol having a molecular weight of 1,025,polyoxytetramethylene glycol, polyoxyhexamethylene glycol,polyoxynonamethylene glycol, polyoxydecamethylene glycol,polyoxydodecamethylene glycol and mixtures therof. Other types ofpolyoxyalkylene glycol ethers can likewise be used. Particularlysuitable polyether polyols are those which are obtained by reacting suchpolyols as ethylene glycol, diethylene glycol, triethylene glycol,1,4-butanediol, 1,3-butanediol, 1,6-hexanediol and mixtures thereof,glyceroltrimethylolethane, trimethylolpropane, 1,2,6-hexanetriol,dipentaerythritol, tripentaerythritol, polypentaerythritol,methylglucosides and sucrose with alkylene oxides, such as ethyleneoxide, propylene oxide or mixtures thereof.

To block the polyisocyanates it is possible to use any desired suitablealiphatic, cycloaliphatic or aromatic alkyl monoalcohols. Examplesthereof are aliphatic alcohols, such as methyl, ethyl, chloroethyl,propyl, butyl, amyl, hexyl, heptyl, octyl, nonyl, 3,3,5-trimethylhexyl,decyl and lauryl alcohol, aromatic alkyl alcohols, such asphenylcarbinol and methylphenylcarbinol. It is also possible to includesmall amounts of higher molecular weight and relatively involatilemonoalcohols, if desired, these alcohols, after detachment, functioningas plasticizers in the coatings. Other suitable blocking agents areoximes, such as methyl ethyl ketone oxime, acetone oxime andcyclohexanone oxime, as well as caprolactams, phenols and hydroxamicacid esters. Preferred blocking agents are malonic esters, acetoaceticesters and β-diketones.

The blocked polyisocyanates are prepared by reacting a sufficient amountof an alcohol with the organic polyisocyanate, so that no freeisocyanate groups are present.

The basecoating compositions according to the invention may contain anyknown pigment or dye customary in the paint industry.

Examples of dyes and pigments which may be of inorganic or organicnature are titanium dioxide, graphite, carbon black, zinc chromate,strontium chromate, barium chromate, lead chromate, lead cyanamide, leadsilicochromate, zinc oxide, cadmium sulfide, chromium oxide, zincsulfide, nickel titanic yellow, chromium titanic yellow, iron oxide red,iron oxide black, ultramarine blue, phthalocyanine complexes, naphtholred, quinacridones, halogenated thioindigo pigments or the like.

Particularly preferred pigments are metal powders alone or a mixture,such as copper, copper alloys, aluminum and steel, preferably aluminumpowder, used in an not less than predominant amount, namely in an amountof from 0.5 to 25% by weight, based on the total solids content ofbinders in the coating agents. Preferred metallic pigments are thosecommercially available metal powders which have been specificallypretreated for aqueous systems.

The metal powders can also be used together with one or more of theabovementioned non-metallic pigments or dyes. In this case, the amountthereof is chosen in such a way that the desired metallic effect is notsuppressed.

The basecoating compositions according to the invention can also containfurther customary additives such as solvents, fillers, plasticizers,stabilizers, wetting agents, dispersants, flow control agents, antifoamsand catalysts individually or in mixture in the customary amounts. Thesesubstances can be added to the individual components and/or to themixture as a whole.

Examples of suitable fillers are talcum, mica, kaolin, chalk, quartzpowder, asbestos powder, slate powder, barium sulfate, various silicas,silicates, glass fibers, organic fibers and the like.

The coating compositions described above are used according to theinvention in processes for preparing multilayered coatings on substratesurfaces, wherein

(1) an aqueous dispersion is applied as a basecoating composition

(2) from which a polymer film is formed on the substrate surface

(3) to the basecoat thus obtained is applied a suitable transparenttopcoat composition and subsequently

(4) the basecoat is baked together with the topcoat.

Suitable topcoat compositions are in principle all known nonpigmented ormerely transparently pigmented coating agents. They may be conventionalsolvent-containing clearcoats, water-dilutable clearcoats or powderclearcoats.

Suitable substrates to be coated are in particular pretreated metalsubstrates, but it is also possible to coat the nonpretreated metals andany other desired substrates such as, for example, wood, plastics andthe like with a multilayered protective and/or decorative coating usingthe basecoating compositions according to the invention.

The invention is explained in more detail in the following examples.Parts and percentages are by weight, unless otherwise stated.

Preparation of a Polyurethane Dispersion According to the Invention

255 g of a polyester of 1,6-hexanediol and isophthalic acid with anaverage molecular weight of 614 are heated together with 248 g ofpolypropylene glycol having an average molecular weight of 600 and 100 gof dimethylpropionic acid to 100° C. and dehydrated for 1 hour underreduced pressure. At 80° C. 526 g of 4,4'-dicyclohexylmethanediisocyanate and 480 g of methyl ethyl ketone are added. Stirring iscontinued at 80° C. until the free isocyanate group content is 1.69%,based on the total starting weight.

28.5 g of trimethylolpropane are then added, followed by 0.4 g ofdibutyltin dilaurate, and 2 hours of stirring at 80° C. After additionof 1,590 g of methyl ethyl ketone the temperature of 80° C. ismaintained until the viscosity, measured in the DIN cup, is 65 s (sampledissolved in a ratio of 2:3 in N-methylpyrrolidone).

After addition of a mixture of 22.4 g of dimethylethanolamine and 2,650g of deionized water, methyl ethyl ketone is distilled off under reducedpressure, to leave a finely divided dispersion having a solids contentof 30%, a pH of 7.4 and a viscosity of 48 s, measured in the DIN cup.

Preparation of Two-Layer Coatings by the Basecoat/Clearcoat Method Usingthe Polyurethane Dispersion According to the Invention Prepared by theAbove Method

The two-layer coating was prepared in accordance with the experimentaldirections given in U.S. Pat. No. 4,558,090.

It proved possible to process a metallic effect basecoat prepared usingthe polyurethane dispersion according to the invention into ahigh-quality two-layer metallic effect coating giving an excellentmetallic effect.

The removability of paint residues remaining behind in the applicationequipment is demonstrated by means of the following comparativeexperiment: A glass plate was knife-coated in a wet film thickness of100 μm with a metallic effect basecoat in accordance with U.S. Pat. No.4,558,090 and a metallic effect basecoat in accordance with the presentinvention.

After 2 hours of drying at room temperature a mixture of 50 parts ofwater and 50 parts of n-propanol was used to try to remove the driedpaint film from the glass plate by guiding under slight pressure a brushwhich had been impregnated with this cleaning mixture in circularmovements over the dried paint film. The paint film formed from themetallic effect basecoat prepared according to the invention dissolvedhomogeneously after the first few circular movements. The paint filmprepared using the dispersion disclosed in U.S. Pat. No. 4,558,090, bycontrast, first became swollen and only became detached from thesubstrate in relatively large connected flakes after many circularmovements.

This experiment was repeated with a large number of different solventand solvent mixtures. Similar results were obtained in all cases.

We claim:
 1. A basecoating composition for preparing a multi-layered,protective or decorative coating, comprising an aqueous dispersion whichcontains:(a) as a film-forming material a polyurethane resin having anacid number of from 5 to 70, which has been prepared from(A) linearpolyetherdiol or polyesterdiol having a molecular weight of from 400 to3,000, (B) diisocyanate and (C) a compound which contains two groupswhich are reactive toward isocyanate groups,component C having one ormore groups capable of anion formation which have preferably beenneutralized before the reaction with a tertiary amine, and components(A), (B) and (C) forming an intermediate which has terminal isocyanategroups and whose free isocyanate groups have subsequently been reactedwith (D) further compounds which contain groups reactive towardisocyanate groups, and (b) pigments,wherein the aqueous polyurethanedispersion has been prepared by (1) reacting the intermediate obtainedfrom (A), (B) and (C) with a polyol which contains three or morehydroxyl groups and (2) transferring the reaction product thus obtainedinto an aqueous phase.
 2. A process for preparing a multi-layered,protective or decorative coating on a substrate surface comprising:(1)applying as a basecoating composition an aqueous dispersion whichcontains(a) a film-forming polyurethane resin having an acid number offrom 5 to 70, which has been prepared from(A) linear polyetherdiol orpolyesterdiol having a molecular weight of from 400 to 3,000, (B)diisocyanate, and (C) a compound which contains two groups which arereactive toward isocyanate groups, component C having one or more groupscapable of anion formation which have preferably been neutralized beforethe reaction with a tertiary amine, components (A), (B) and (C) formingan intermediate which has terminal isocyanate groups and whose freeisocyanate groups have subsequently been reacted with (D) a compoundwhich contains groups reactive toward isocyanate groups, and (b)pigments; (2) forming from the composition applied in stage (1) apolymer film on the surface; (3) applying to the basecoat thus obtaineda suitable transparent topcoat composition, and (4) baking the basecoattogether with the topcoat,wherein the polyurethane dispersion formingthe basecoating composition has been prepared by reacting theintermediate obtained from (A), (B) and (C) with a polyol which containsthree or more hydroxyl groups.
 3. A basecoating composition as describedin claim 1 wherein the reaction of the intermediate obtained from (A),(B) and (C) with the polyol has been carried out in a water-miscibleorganic solvent which boils below 100° C., is inert toward isocyanategroups and is readily separable from aqueous mixtures.
 4. A basecoatingcomposition as described in claim 1 wherein the intermediate obtainedfrom (A), (B) and (C) has been reacted with trimethylolpropane.
 5. Abasecoating composition as described in claim 1 wherein the basecoatingcomposition contains as a binder component a water-dilutable melamineresin in an amount from 1 to 80% by weight, based on the solids contentof the polyurethane dispersion.
 6. A basecoating composition asdescribed in claim 5 wherein the basecoating composition contains as afurther binder component a water-dilutable polyester resin or awater-dilutable polyacrylate resin, the weight ratio of melamine resin:polyester resin or polyacrylate resin being from 2:1 to 1:4 and thetotal proportion of melamine resin, polyester resin and polyacrylateresin, based on the solids content of the polyurethane dispersion,ranging from 1 to 80% by weight.
 7. A basecoating composition asdescribed in claim 1 wherein the basecoating composition contains as abinder component a blocked polyisocyanate together with awater-dilutable polyester resin or a water-dilutable polyacrylate resin,the proportion of polyisocyanate, polyester resin or polyacrylate resinas a whole ranging from 1 to 80% by weight, based on the solids contentof the polyurethane dispersion.
 8. A basecoating composition asdescribed in claim 1 wherein the basecoating composition contains metalpigments in an amount ranging from 0.5 to 25% by weight, based on thetotal solids content of the polyurethane dispersion.
 9. A substratecoated with a multi-layered, protective or decorative coating which hasbeen obtained by(1) applying as a basecoating composition an aqueousdispersion which contains(a) as a film-forming material one or morepolyurethane resins having an acid number of from 5 to 70, which hasbeen prepared from(A) linear polyetherdiol or polyesterdiol having amolecular weight of from 400 to 3,000; (B) diisocyanate, and (C) acompound which contains two groups which are reactive toward isocyanategroups, component C having one or more groups capable of anion formationwhich have preferably been neutralized before the reaction with atertiary amine, components (A), (B) and (C) forming an intermediatewhich has terminal isocyanate groups and whose free isocyanate groupshave subsequently been reacted with (D) further compounds which containgroups reactive toward isocyanate groups, and (b) pigments; (2) formingfrom the composition applied in (1) a polymer film on the surface; (3)applying to the basecoat thus obtained a suitable transparent topcoatcomposition, and (4) baking the basecoat together with thetopcoat,wherein the polyurethane dispersion forming the basecoatingcomposition has been prepared by reacting the intermediate obtained from(A), (B) and (C) with a polyol which contains three or more hydroxylgroups and transferring the reaction product thus obtained into anaqueous phase.
 10. The basecoating composition of claim 1 wherein thepolyol which contains three or more hydroxyl groups is a triol.
 11. Theprocess of claim 2 wherein the polyol which contains three or morehydroxyl groups is a triol.
 12. The basecoating composition of claim 3wherein the water-miscible organic solvent is acetone.
 13. The processof claim 2 wherein the intermediate obtained by reacting (A), (B) and(C) with the polyol is carried out in a water-miscible organic solventwhich boils below 100° C., is inert toward isocyanate groups and isreadily separable from aqueous mixtures.
 14. The process of claim 13wherein the water-miscible organic solvent is acetone.
 15. The processof claim 2 wherein the intermediate obtained from (A), (B) and (C) isreacted with trimethylolpropane.
 16. The process of claim 2 furthercomprising adding to the basecoating composition a binder componentcomprised of a water dilutable melamine resin in an amount ranging from1 to 80% by weight, based on the solids content of the polyurethanedispersion.
 17. The process of claim 16 where the water dilutablemelamine resin is present in an amount ranging from 20 to 60% by weight.18. A basecoating composition as described in claim 5 wherein themelamine resin is present in an amount ranging from 20 to 60% by weight,based on the solids content of the polyurethane resin.
 19. A process asdescribed in claim 16 further comprising adding to the basecoatingcomposition a water-dilutable polyester resin or a water-dilutablepolyacrylate resin, the weight ratio of melamine resin: polyester resinor polyacrylate resin being from 2:1 to 1:4, and the total proportion ofmelamine resin, polyester resin and polyacrylate resin, ranging from 1to 80% by weight based on the solids contents of the polyurethanedispersion.
 20. A process as described in claim 19 wherein the totalproportion of melamine resin, polyester resin and polyacrylate resinranges from 20 to 60% by weight based on the solids content of thepolyurethane dispersion.
 21. A basecoating composition as described inclaim 6 wherein the total proportion of melamine resin, polyester resinand polyacrylate resin ranges from 20 to 60% by weight, based on thesolids content of the polyurethane dispersion.
 22. The process describedin claim 2 further comprising adding a blocked polyisocyanate togetherwith a water dilutable polyester resin or polyacrylate resin, theproportion of polyisocyanate, polyester resin or polyacrylate resin,ranging from 1 to 80% by weight, based on the solids content of thepolyurethane dispersion.
 23. The process described in claim 2 whereinthe basecoating composition contains metal pigment in an amount rangingfrom 0.5 to 25% by weight, based on the total solids content of thepolyurethane dispersion.